In recent years, an optical functional device on a silicon substrate using a silicon electronic circuit manufacturing technology which is low in cost and capable of large-scale integration has been attracted attention. In a high performance server, a super computer, and so on, high computing power was realized by a multi-core CPU or the like to meet the demand of increasing in required processing power. On the other hand, conventional electrical transmission between chips and between boards is going to face the limit because of the signal speed and transmission length. A large-scaled optical communication device on a silicon substrate based on a low-loss and small-sized silicon-wire waveguide, so-called a silicon photonics is expected as a technology to solve problems of lack of communication capacity of an above-stated speeding up information processing device. Especially, an application of a wavelength division multiplexer (WDM) technology which is practically used in telecommunication system to the silicon photonics is expected to have an effect on enabling high density of transmission capacity and reduction in optical cables, and research and development thereof have been widely advanced.
In an optical transmitter/receiver using the silicon photonics, an optical transmitter is made up of a light source, an optical modulator, and so on, and an optical receiver is made up of a photodetector, or the like. Optical fibers connect between the optical transmitter and the optical receiver. In this case, a signal light input to the optical receiver has an irregular polarization state resulting from a change of a stress applied to the optical fiber and a change of a temperature. This becomes particularly a large problem in a WDM transmission system which requires an optical demultiplexer or the like whose polarization dependence is large.
For example, a polarization diversity optical receiver as illustrated in FIG. 15 (or refer to Non-Patent Document 1) has been proposed to deal with the above-stated problem.
In this polarization diversity optical receiver, when a WDM input signal light which is in the irregular polarization state is input to a polarization beam splitter 101, the signal light is separated into a TE polarization light and a TM polarization light. The TM polarization light is input to a polarization rotator 102 to be rotated to the TE polarization light. The signal lights separated into two lights as stated above are each input to an optical demultiplexer as the TE polarization light. A ring optical resonator or an AWG filter is used as the optical demultiplexer, but here, a bidirectional input type ring optical resonator 103 is exemplified as the optical demultiplexer. The two TE polarization lights split into separated lights are input to the ring optical resonator 103 from opposite directions, the TE polarization light matching a resonant wavelength of the ring optical resonator 103 is coupled to a drop port 104 which is optically coupled to the ring optical resonator 103, and input to a photodetector 105 formed next thereto. As stated above, the WDM input signal light is once separated into the TE polarization light and the TM polarization light, and further, the TM polarization light is rotated into the TE polarization light, and thereby, an optical signal receiving which is independent from the polarization state of the input signal light is enabled.
[Patent Document 1] Japanese Laid-open Patent Publication No. 2010-287623
[Patent Document 2] Japanese Laid-open Patent Publication No. 2010-91900
[Non-Patent Document 1] Long Chen, Christopher R. Doerr, and Young-kai Chen, OFC/NFOEC Technical Digest, 2012
[Non-Patent Document 2] Lucas B. Soldano and Erik C. M. Pennings, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 13, NO. 4, APRIL 1995
As stated above, the polarization diversity optical receiver is used, and thereby, an optical signal receiving which is independent from a polarization state of an input signal light can be obtained, but the signal lights are input from both directions of the photodetector 105 according to the structure as illustrated in FIG. 15. Therefore, if a photodetector length is not set to be enough long, there is a problem in which transmission light is output to each facing input port, the output transmission light is output as an optical feedback toward inside/outside the optical receiver via the ring optical resonator 103, and causes noises and malfunctions. On the other hand, when the photodetector length is set to be long so that the transmission light from the photodetector 105 becomes enough small, there is a problem in which a capacitance increases and a frequency response bandwidth is deteriorated.